Here’s a most excellent demonstration of how to author a custom design experience that’s shared between WPF and Silverlight controls. Karl gives you a detailed, step-by-step walkthrough, complete with Task List breadcrumbs:

BreadcrumbTrail

Walkthrough guidance in the form of Task List hints

That’s so clever it blows my mind. Karl shows you precisely how to isolate and package design-time logic that’s specific to WPF and Silverlight, as well as VS and Blend – the table showing which design-time assemblies are loaded in which design tool (VS or Blend) is worth the price of entry by itself. And he shows you the key to code sharing: how to abstract SL- and WPF-specific types through a little type resolver class. It’s all very slick.

Karl’s blog post is here, and the code is here at WPF and Silverlight Designer Extensibility Samples. Have fun with your custom design experiences.